1. Field of the Invention
The present invention relates to a process and an apparatus for a heat treatment of a polymer containing liquid, more particularly, to a process and an apparatus for a heat treatment of a polycarbonate solution wherein deterioration in color is eliminated or reduced in a process for recovering a polycarbonate resin dissolved in a chlorinated organic solvent.
2. Description of the Related Art
As an industrial process for manufacturing a polycarbonate, a phosgene method and a melt transesterification method are generally known. The melt transesterification method has a disadvantage of color deterioration of a product, because of a prolonged treatment at an elevated temperature. Thus, the phosgene method is prevailingly employed.
After washing the polycarbonate solution (polymer solution) produced by polymerization, the phosgene method requires steps of concentration (flaking), drying (removing a solvent), and then, granulation (pelletizing), in separate apparatuses. Of those steps, the concentration and drying steps are severe burdens. Large equipment such as a kneader or a dryer is needed, and an operation cost is too expensive.
Accordingly, it is desired to simplify the above steps. Various processes for recovering a polymer from a polymer solution were proposed.
For example, a process for isolating the polymer from the polymer solution by adding a poor solvent is described in Japanese Examined (KOKOKU) Patent Publications No. 37-5599, No. 37-18399, No. 39-1959, No. 42-14474 and so on. Japanese Examined (KOKOKU) Patent Publication No. 53-15899 describes a process for gelling and grinding the polymer solution by a kneader or the like. Japanese Unexamined (KOKAI) Patent Publication No. 60-115625 discloses a process for obtaining the polymer in the form of a slurry by bringing the polymer solution into contact with a hot water to remove the solvent with a steam. The above processes have both advantages and disadvantages. Each process needs a number of steps, and costs of an initial construction of equipment and operation thereof are very expensive.
As a simplified and economical process for manufacturing polycarbonate to remedy the above disadvantages, Japanese Unexamined (KOKAI) Patent Publication No. 62-183801 discloses a process for producing polycarbonate in a melted state by heating the polymer solution with a specific heat exchanger to remove the solvent. However, the process has disadvantages of coloring of polycarbonate by a heat deterioration, and a residual solvent in a recovered polycarbonate.
It is well known that polycarbonate is characterized by high impact properties and good transparency. Thus, reduction of transparency, i.e., color deterioration by heat deterioration can be a considerable defect. The problem of heat deterioration may be solved by lowering a temperature at a heat desolvation as low as possible, and shortening a heating time as short as possible. Under such conditions, however, an amount of the residual solvent is increased. For sufficiently removing the residual solvent, it is necessary to raise the temperature of the heat desolvation and prolong the desolving time. Therefore, it is difficult to obtain polycarbonate with good transparency and without the residual solvent, by the above-mentioned process.
An apparatus, such as an extruder, wherein a polymer is treated at a temperature above the glass transition temperature thereof (Tg) may generally encounter the coloring problem of the polymer by heat deterioration. If a chlorinated organic solvent, such as methylene chloride, which may be preferably used as a solvent for polycarbonate, or metallic impurities are contained in a great amount, the polymer are considerably affected thereby. It is believed that the chlorinated organic solvent is decomposed to produce hydrochloric acid, which forms a metallic chloride upon contacting with a treating apparatus. The metallic chloride serves as a catalyst to accelerate deterioration and coloring of the polymer. The above phenomenon is a general problem in a polyester resin (in a broad sense) having oxygen containing ester bonds. In particular, polycarbonate is tremendously affected. The metallic impurities are believed to enter from the apparatus used in a high temperature treatment, by a decomposing action of the solvent.
An influence of the decomposition of the chlorinated organic solvent can be avoided by employing a material inactive to the produced hydrochloric acid, such as an inorganic material (for example, glass or ceramics), or a precious metal (for example, gold) which has an anticorrosive property in a practical use. However, it was considered difficult to realize the use of such a material, because of a cost, a strength, and a construction problem, and further, in the case of the inorganic material (for example, glass or ceramics), heat transmission.